Known in the art is an acoustic wave element having a piezoelectric substrate and an IDT (InterDigital Transducer) provided on a major surface of the piezoelectric substrate. Such an acoustic wave element is for example utilized in a transmitting filter or receiving filter of a duplexer or the like.
In the acoustic wave element, sometimes electrical characteristics fall due to electrical distortion waves generated due to the nonlinearity of the element. For example, in a duplexer using an acoustic wave element, a disturbance wave out of the transmission band and reception band and a transmission wave are mixed whereby a distortion wave included in the reception band is generated. This distortion wave is called “inter-modulation distortion” (IMD) and is one of the causes of a drop in the quality of communication (SN ratio) of wireless communications. Other than this, there also exists a possibility of the problem that harmonic distortion having a frequency of a whole multiple of the transmission wave will be generated and that this will disturb communications of other wireless apparatuses.
Therefore, in order to suppress a drop in the SN ratio due to a distortion wave, a method of dividing a resonator without changing the electrostatic capacitance is known (for example Japanese Patent Publication No. 2007-074698A). Resonators to be divided are serial resonators or parallel resonators in a ladder type filter which configure a duplexer. The voltage applied to resonators are dispersed by dividing, and therefore, a distortion wave is suppressed.
Note that, while not literature related to the art of suppressing a distortion wave, Japanese Patent Publication No. 5-167384A discloses a capacity element which is provided on a major surface of a piezoelectric substrate and is connected in parallel to the IDT.
However, if resonators are divided without changing the electrostatic capacitance, the resonators become larger in size compared with before division. Consequently, the acoustic wave element becomes larger in size. Further, the technique of dividing the resonators can be applied to a ladder type filter, but cannot be applied to the multi-mode type filter which is generally employed in a receiving filter.